1. Field of the Invention
The present invention relates to a solenoid valve, which has a slider (plunger) supported in a reciprocable manner. More specifically, the present invention relates to a solenoid-operated oil pressure control valve, which selectively supplies and drains oil pressure of an oil pressure source relative to a retarding oil pressure chamber(s) and an advancing oil pressure chamber(s) of an intake/exhaust variable valve timing mechanism that changes a valve timing of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 10-38126, German Unexamined Patent Publication No. 19504185 and Japanese Unexamined Patent Publication No. 11-118062 disclose a solenoid valve, which has a slider (plunger) supported in a reciprocable manner. The solenoid valve further includes a coil, a spring, a stator and a slider supporting member. The coil generates magnetomotive force for attracting the slider in one of reciprocating directions of the slider. The spring provides urging force for urging the slider in the other of the reciprocating directions of the slider. The stator forms a magnetic circuit in cooperation with the slider and has an attracting portion for attracting the slider. The slider supporting member is arranged radially inward of the stator and supports the slider in the reciprocable manner.
Furthermore, Japanese Unexamined Patent Publication No. 2001-187979 also discloses such a solenoid valve. In the solenoid valve disclosed in Japanese Unexamined Patent Publication No. 2001-187979, a distal end of a slider (plunger) is tapered. Because of the tapered distal end of the slider, a contact surface area between the slider and a cup-shaped non-magnetic body (plunger guide) is reduced, and thus larger friction causing wearing is generated in the contact surface between the slider and the non-magnetic body. Furthermore, the tapered distal end of the slider provides a greater chance of introducing debris and dusts between the slider and the non-magnetic body, causing improper sliding operation of the slider.
Furthermore, the non-magnetic body is made of a thin stainless steel to improve a magnetic efficiency. Thus, if a wall thickness of the non-magnetic body is increased to improve strength of the non-magnetic body, the magnetic efficiency is reduced. Because of this, the non-magnetic body needs to have the thin wall in order to provide stable attractive force in the slider although it causes an increased possibility of damaging the non-magnetic body by the friction generated between the slider and the non-magnetic body. In order to reduce the friction between the slider and the non-magnetic body, the taper can be removed from the tapered distal end of the slider. However, the removal of the taper from the tapered distal end of the slider causes reduction of the stable attractive force in the slider.
Furthermore, Japanese Unexamined Patent Publication number 61-244984 and German Unexamined Patent Publication No. 19504185 disclose another solenoid valve. The solenoid valve includes a valve member, which is reciprocated together with a slider to open and close fluid passages or to increase and decrease a flow of fluid in the fluid passages.
Each solenoid valve disclosed in Japanese Unexamined Patent Publication number 61-244984 and German Unexamined Patent Publication No. 19504185 includes a relief passage provided in the slider. The relief passage communicates between spaces, which are arranged adjacent to opposite axial ends of the slider, respectively. Thus, the communication passage can restrain development of an excessive pressure difference between the spaces and allows smooth axial movement of the slider.
Specifically, Japanese Unexamined Patent Publication No. 61-244984 discloses the relief passage in a form of a spiral groove, which extends along an outer peripheral surface of the slider between the axial ends of the slider. German Unexamined Patent Publication No. 19504185 discloses the relief passage in a form of an axial passage, which extends between the axial ends of the slider near an outer peripheral surface of the slider. With such relief passages, it is possible to restrain generation of the excessive pressure difference between the spaces provided on opposite axial ends of the slider, respectively.
However, the relief passage disclosed in Japanese Unexamined Patent Publication No. 61-244984 and the relief passage disclosed in German Unexamined Patent Publication No. 19504185 are both provided at or near the outer peripheral surface of the slider, so that a magnetic resistance in the slider, which constitutes part of a magnetic circuit, is increased, and thus a magnetic permeability of the slider is reduced, causing a reduction in magnetic attractive force for attracting the slider.